The present invention is directed to a system for controlling emission of a solvent from a printing press having a plurality of printing stations with each station being provided with a drying hood which is connected to an exhaust manifold going to an oxidation chamber for oxidizing the solvents from the drying gas with the exhaust of the oxidation chamber being recirculated back for use in the drying hoods.
In a printing press, after the ink solution has been deposited on the web, the web is passed through a dryer. The dryer will include an enclosure or hood in which a heated air or gas is passed over the web to evaporate the solvent from the depositing solution. Known structures of the dryer with the various nozzle configurations are disclosed in U.S. Pat. Nos. 2,554,239, 2,791,039, and 2,932,092.
The exhaust from each of these drying devices or hoods will include a volatile organic compound of the solvent which are removed during the drying process. Air pollution standards require that the solvents present in the exhaust gases from the dryers of the press be removed prior to releasing the exhaust gases into the environment. To remove the volatile organic compounds from the exhaust, two different types of approaches have been suggested. One is to attempt to recover the solvents, however, this recovery is limited to only certain types of solvents. The second method is to remove the solvents by heating the exhaust gases to a temperature of about 1400.degree. F. If the gases are held at this temperature for approximately 0.6 seconds, about 99% of the solvents are converted into harmless water vapor and carbon dioxide. An example of a system and device for thermal oxidation of the volatile organic compounds in the gases is disclosed in U.S. Pat. No. 4,176,162.
Since the energy requirements to operate conventional pollution control system in which the volatile organic compound are oxidized in an oxidation chamber or incinerator such as disclosed by U.S. Pat. No. 4,176,162 are a direct function of the amount of air processed through the device for clean-up, these devices utilize a lower explosive limit (LEL) control unit. The lower explosive limit control unit measures the concentration of the volatile organic compounds in the dryer exhaust at each hood and recirculates the dryer exhaust back through the nozzle with a mixture of fresh air so that the volatile organic concentration levels are maximized but do not exceed a level at which a danger of explosion would occur. Thus, the amount of the volatile organic compounds return to the atmosphere is a function of the following two factors or variables of the process which are the clean up efficiency of the oxidation device, and the amount of fresh air introduced into the process to control the concentration of the volatile organic compounds at the point of the exhaust from the dryer hoods. This second factor is critical because the basic system requires that all fresh air introduced into the system must be returned to the atmosphere after processing through the dryer hoods or the like in order to maintain a balanced system.
In the drying hood of a conventional system, the normal air flow at the drying hood is to withdraw the air required for drying from the hood, heat the withdrawn air and return it through the drying nozzles back into the hood. The heat of this case can either be from an external source or by mixing the hood air with hot air from the exhaust of the volatile organic compound reduction device. In either case, a small amount of air is withdrawn from each hood to maintain a system balance. Should the volume of the volatile organic compounds emitted from one hood be large, an LEL control unit, such as disclosed in U.S. Pat. No. 4,150,495, will measure the concentration and increase the volume of fresh air being introduced into the hood. Such an increase in the volume of fresh air to the system will increase the volume of air that must be returned to the atmosphere from the system.
The conventional pollution control system such as disclosed in the above mentioned U.S. Pat. No. 4,176,162, when used with rotogravure packaging industry has a clean-up efficiency of between 90%-95%. This efficiency is calculated by measuring the concentration of the volatile organic compounds in the dryer air stream leaving the basic process and comparing that concentration to the concentration of volatile organic compounds leaving the pollution control device and being emitted into the atmosphere. The current trend in pollution control legislation is to define the limit on the amount of the pollutants that can be emitted into the atmosphere whereas in the past, the requirements are usually pointed at a percentage clean-up or reduction in the total emissions. Therefore, if the user of a volatile organic compound is to satisfy current environmental protection agency legislation, the user can either limit his usage to the point where the conventional system can satisfy the total emission requirements or the user must obtain a new system.